氮在钌单晶表面Ru(0001)上的吸附

钌系氨合成催化剂作为第二代氨合成催化剂,近年来日益受到重视。研究氮分子在钌表面上的离解吸附对提高钌催化剂的催化性能有着十分重要的意义。综述了氮分子在Ru(0001)单晶表面吸附的研究进展，并指出钌催化剂的发展方向。     

Spill-Over Effects on Bimetallic Pt/Ru(0001) Surfaces

The concept of spill-over of adsorbed species has a long tradition in Heterogeneous Catalysis and has been explored also for adsorption on bimetallic surfaces, in particular by the Goodman group. In the present paper, we report results of a comprehensive temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy study on spill-over effects in the adsorption and desorption of CO on structurally well defined bimetallic Pt/Ru(0001) surfaces, where part of the substrate is covered by monolayer Pt islands. While upon adsorption at 90 K, the mobility of CO_ad molecules on the surface is very limited, it is activated when the adlayer is annealed to 150 K or, more directly, if CO exposure is done at 150 K or higher temperatures. This enables diffusion of CO_ad molecules to the Pt free Ru(0001) areas, even at local CO_ad coverages which preclude further adsorption from the gas phase on the Ru parts of the surface. Spill-over processes are shown to have significant impact on the TPD spectra; furthermore they provide an additional adsorption channel for adsorption on the bare Ru(0001) areas, allowing uptake of CO at local coverages where adsorption from the gas phase is precluded. This indicates that the apparent CO saturation coverage of 0.68 ML determined for direct adsorption on Ru(0001) under UHV conditions is limited by kinetics rather than thermodynamics. The data are discussed in comparison with results and interpretations in earlier studies, which indicate that these effects are not limited to the Pt/Ru(0001) surface, but may be found on a wide range of bimetallic systems.     

Synthesis of high-quality monolayer and bilayer graphene on copper using chemical vapor deposition

The mechanisms determining the growth of high-quality monolayer and bilayer graphene on Cu using chemical vapor deposition (CVD) were investigated. It is shown that graphene growth on Cu is not only determined by the process parameters during growth, but also substantially influenced by the quality of Cu substrate and how the Cu substrate is pretreated. It is found that the micro-topography of the Cu surface strongly affects the uniformity of grown graphene while the purity of the Cu film determines the number of synthesized graphene layers at low pressure conditions. On the other hand, a minimum partial pressure of hydrocarbon is required for graphene to cover the Cu surface during graphene growth. The optimized bilayer graphene exhibits a maximum hole (electron) mobility of 5500 cm²V^–1s^–1 (3900 cm²V^–1s^–1). A new growth mode resulting in tetragonal shaped graphene domain, which is different from the known lobe structure (for monolayer) or hexagonal (for few layer) mode, is also discovered under our experimental conditions. Furthermore, high resolution transmission electron microscopy has revealed the non-ideal nature of CVD graphene structure for the first time, indicating an important cause of electron/hole mobility degradation that is typically observed in CVD graphene. This observation could be crucial for optimization of the CVD process to further improve the quality of graphene.     

Magnetic properties of double-side partially fluorinated graphene from first principles calculations

Density functional theory calculations were used to study the structural, electronic, and magnetic properties of double-side partially fluorinated graphene. Both even and uneven fluorinated structures examined. It is found that midgap states and magnetic moments only appear in the uneven double-side fluorinated graphene. The magnetic moments mainly come from the carbon atoms at the edge of the fluorinated region in unevenly double-side fluorinated graphene. The dependence of magnetic moments on external tensile strain was also examined, which shows that the induced magnetic moments can be significantly increased by increasing the tensile strain.     

The characterization of carbonaceous species from CO hydrogenation on single crystal Ru(0001) and Ru(11¯20) catalysts with high-resolution electron energy-loss spectroscopy

The CO hydrogenation on single-crystal ruthenium catalysts has been studied utilizing an elevated-pressure micro-reactor and high resolution electron energy loss spectroscopy (HREELS). It is found that carbonaceous deposits identified following CO hydrogenation are essentially identical to those observed in the study of methane decomposition. Three distinct forms of carbonaceous intermediates are identified; these are methylidyne (CH), vinylidene (CCH₂), and graphitic carbonaceous species.     

Decomposition of Ethanol Over Ru(0001): A DFT Study

We studied decomposition pathways of ethanol on Ru(0001) with periodic slab-model calculations using a DFT-GGA approach. We calculated the adsorption modes of ethanol and several of its dehydrogenation products and we evaluated reaction energies as well as activation barriers of pertinent dehydrogenation, C–C, and C–O cleavage steps. The calculated barrier heights of C–C and C–O scission steps can be related to the number of hydrogen atoms bound to the C1–C2 and C1–O moieties of the intermediates, respectively. Two counteracting effects are at work, increasing with each dehydrogenation: (i) higher order of the pertinent bond of the adsorbate, and (ii) stronger substrate-surface interaction and thus better stabilization of the transition state. For most intermediates we determined C–O cleavage to be both kinetically and thermodynamically favored over C–C scission, except for the highly dehydrogenated species CH _k CO (k = 1, 2). Based on the calculated energetics, the most likely decomposition pathway, with a rate-determining barrier at 77 kJ·mol^?1, leads to the formation of ketene CH₂CO and subsequent C–C cleavage yielding methylene and CO.     

Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0 0 0 1) layers

Quasi-free-standing monolayer and bilayer graphene is grown on homoepitaxial layers of 4H-SiC. The SiC epilayers themselves are grown on the Si-face of nominally on-axis semi-insulating substrates using a conventional SiC hot-wall chemical vapor deposition reactor. The epilayers were confirmed to consist entirely of the 4H polytype by low temperature photoluminescence. The doping of the SiC epilayers may be modified allowing for graphene to be grown on a conducing substrate. Graphene growth was performed via thermal decomposition of the surface of the SiC epilayers under Si background pressure in order to achieve control on thickness uniformity over large area. Monolayer and bilayer samples were prepared through the conversion of a carbon buffer layer and monolayer graphene respectively using hydrogen intercalation process. Micro-Raman and reflectance mappings confirmed predominantly quasi-free-standing monolayer and bilayer graphene on samples grown under optimized growth conditions. Measurements of the Hall properties of Van der Pauw structures fabricated on these layers show high charge carrier mobility (>2000 cm²/Vs) and low carrier density (<0.9 × 10¹³ cm⁻²) in quasi-free-standing bilayer samples relative to monolayer samples. Also, bilayers on homoepitaxial layers are found to be superior in quality compared to bilayers grown directly on SI substrates.     

Transport in suspended monolayer and bilayer graphene under strain: A new platform for material studies

We develop two types of graphene devices based on nanoelectromechanical systems (NEMS), that allows transport measurement in the presence of in situ strain modulation. Different mobility and conductance responses to strain were observed for single layer and bilayer samples. These types of devices can be extended to other 2D membranes such as MoS₂, providing transport, optical or other measurements with in situ strain.     

Nanoscale structural characterization of epitaxial graphene grown on off-axis 4H-SiC (0001)

In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (T _gr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each T _gr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.     

Improved Properties of the Catalytic Model System Ni/Ru(0001)

The dissociative chemisorption of CH₄ on Ni overlayers on Ru(0001) has been investigated. It is found that the initial sticking probability at T=530 K is approximately a factor of 20–30 higher on a pseudomorphic overlayer of Ni than on Ni(111) and a factor of two higher than on Ru(0001) illustrating the unique properties of metal-on-metal systems. The effect of enhanced reactivity is primarily ascribed to electronic effects induced by a straining of the Ni overlayer. The enhanced reactivity towards CH₄ is accompanied by new features in the thermal desorption spectra of CO. The reactivity of the system depends strongly on the annealing temperature. Molecular beam experiments at high translational energy are qualitatively different from thermal data showing a monotonic decrease of the CH₄ sticking probability as Ni is added.     

Selective Excitation of Molecule-Surface Vibrations in H2 and D2 Dissociatively Adsorbed on Ru(0001)

In this contribution we report about the selective vibrational excitation of H₂ and D₂ on Ru(0001) as an example for nonadiabatic coupling of an open quantum system to a dissipative environment. We investigate the possibility of achieving state-selective vibrational excitations of H₂ and D₂ adsorbed on a Ru(0001) surface using picosecond infrared laser pulses. The systems’ behavior is explored using pulses that are rationally designed and others that are optimized using a time-local variant of Optimal Control Theory. The effects of dissipation on the laser-driven dynamics are studied using the reduced-density matrix formalism. The non-adiabatic couplings between adsorbate and surface are computed perturbatively, for which our recently introduced state-resolved anharmonic rate model is used. It is shown that mode- and state-selective excitation can be achieved in the absence of dissipation when using optimized laser pulses. The inclusion of dissipation in the model reduces the state selectivity and the population transfer yield to highly excited states. In this case, mode activation is most effectively realized by a rational pulse of carefully chosen duration rather than by a locally optimized pulse.     

Coadsorption of CO and O on Ru(0001): A Structural Analysis by Density Functional Theory

Knowledge of the atomic geometry of a surface is a prerequisite for any detailed understanding of the surface's electronic structure and chemical properties. Previous studies have convincingly demonstrated that density functional theory (DFT) yields accurate surface atomic geometries and that reliable predictions concerning stable and metastable phases can be made on the basis of the calculated energetics. In the present work, we use DFT to investigate the atomic structure of four ordered coadsorbate phases of carbon monoxide and oxygen on Ru (0001). All of the structures have a (2 × 2) periodicity with differing concentrations of CO molecules and O atoms. For two of these phases dynamical low-energy electron diffraction (LEED) intensity analyses have been performed, and the agreement between our DFT- and the LEED-determined structures is found to be very good. We predict the atomic geometry of the third phase, for which no structural determination based on experiments has been made to date. We also predict the stability of a new ordered mixed phase.     

Methanol in microporous materials from first principles

Zeolites are amongst some of the most important heterogeneous catalysts in use commercially today, combining acid–base catalysis due to the presence of Brønsted acid sites with shape selectivity resulting from the microporous environment [1]. Despite this, there is still a great deal of uncertainty concerning the mechanisms of many of the processes which are known to occur and the way in which the zeolite accelerates them. While much information has been obtained from experimental techniques, including infra-red spectroscopy and magic angle spinning NMR [2], there is presently a need for models and reaction pathways to aid in their interpretation. Here theoretical methods are playing a major role in the field of microporous materials.     

Non-metal catalytic synthesis of graphene from a polythiophene monolayer on silicon dioxide

Direct synthesis of graphene without metal catalysts on a dielectric substrate is a major goal in graphene-based electronics and is an increasingly popular nanotechnology alternative to metal oxide semiconductor technology. However, current methods for the synthesis of these graphenes have many limitations, including the use of metal catalyst. Herein, we report a facile approach to the direct synthesis of graphene sheets based on the self-assembled monolayers (SAMs) technique. The new method for metal catalyst-free direct synthesis of a graphene sheet is through a solution-processable, inexpensive, easy, and reproducible cross-linked polythiophene self-assembled monolayer (SAM) that is formed via the [4 + 2] π cycloaddition reaction of π-electron conjugated thiophene layer self-assembled on the dielectric silicon dioxide substrate. The bifunctional molecules were carefully designed to create an SAM via silanization of alkoxy silane groups on the SiO₂ substrate, and at the other end, a thin cross-linked polythiophene layer via a [4 + 2] π-electron cycloaddition reaction of π-electron conjugated thiophene SAM. By heating the cross-linked polythiophene SAM up to 1000 °C under a high vacuum, single-layered or few-layered graphene sheets were successfully prepared on the dielectric silicon oxide substrate.     

Atomic and Molecular Adsorption on Re(0001)

Using periodic, self-consistent density functional theory calculations, the adsorption of several atomic (H, S, N, O and C) and molecular (CO₂, N₂, NH₃, HCN, CO and NO) species and molecular fragments (NH₂, NH, CN, CNH₂, HNO, NOH, CH₃, CH₂, CH and OH) on the (0001) facet of rhenium at a coverage of 0.25 ML has been studied. Preferred binding sites with their corresponding binding energy and deformation energy of the surface, as well as an estimated diffusion barrier of each species have been determined. Atomic species and molecular fragments tend to bind to threefold sites, whereas molecular species tend to bind to top sites. The binding strength, with respect to the corresponding gas phase species and in increasing order for all species studied, is: CO₂ < N₂ < NH₃ < CO < CH₃ < HCN < NO < H < NH₂ < OH < CH₂ < CNH₂ < CN < HNO < NH < NOH < S < N < O < CH < C. The vibrational frequencies of all species in their most energetically favorable adsorbed configuration have been calculated. Finally, the thermochemistry of adsorption and decomposition of NO, NO + H, NH₃, N₂, CO₂, CO and CH₄ on Re(0001) has been analyzed.     

Chemical vapour deposition of graphene on Ni(111) and Co(0001) and intercalation with Au to study Dirac-cone formation and Rashba splitting

We show in detail monitoring by photoelectron spectroscopy how graphene can be grown by chemical vapour deposition on the transition-metal surfaces Ni(111) and Co(0001) and intercalated by a monoatomic layer of Au. For both systems, a linear E(k) dispersion of massless Dirac fermions appears in the graphene π-band in the vicinity of the Fermi energy. In order to study ferromagnetism and spin-orbit effects by spin- and angle- resolved photoelectron spectroscopy, the sample must be magnetized in remanence. To this end, a W(110) substrate is prepared, its cleanliness verified by photoemission from W(110) surface states and surface core levels, and epitaxial Ni(111) and Co(0001) thin films are grown on top. Spin-resolved photoemission from the π-band shows that the ferromagnetic polarization of graphene/Ni(111) and graphene/Co(0001) is negligible and that graphene on Ni(111) is after intercalation of Au spin-orbit split by the Rashba effect.     

Novel Unsymmetrical Ru(III) and Mixed-valence Ru(III)/Ru(II) Dinuclear Compounds Related to the Antimetastatic Ru(III) Drug NAMI-A

In this paper we report the stepwise preparation and the characterization of new unsymmetrical monoanionic Ru(III) dinuclear compounds, [NH(4)][{trans-RuCl(4)(Me(2)SO-S)}(mu-L){mer-RuCl(3)(Me(2)SO-S)(Me(2)SO-O)}] (L = pyz (1), pym (2)). By a similar synthetic approach we also prepared new mixed-valence Ru(III)/Ru(II) dinuclear compounds of formula [NH(4)][{trans-RuCl(4)(Me(2)SO-S)}(mu-pyz){cis,cis,cis-RuCl(2)(Me(2)SO-S)(2)(CO)}] (L = pyrazine (pyz, 3), pyrimidine (pym, 4)). Moreover, we describe the chemical behavior of compounds 1-4 in physiological solution, also after complete reduction (with ascorbic acid) to the corresponding Ru(II)/Ru(II) species. Overall, the chemical behavior of 1 and 2 after reduction resembles that of the corresponding dianionic and neutral dinuclear species, [{trans-RuCl(3)(Me(2)SO-S)}(2)(mu-L)](2-)and [{mer-RuCl(3)(Me(2)SO-S)(Me(2)SO-O)}(2) (mu-L)]. On the other hand, the mixed-valence dinuclear compounds 3 and 4, owing to the great inertness of the cis,cis,cis-RuCl(2)(Me(2)SO-S)(2)(CO)(1/2mu-L) fragment, behave substantially like the mononuclear species [trans-RuCl(4)(Me(2)SO-S)(L)](-) in which the terminally bonded L ligand can be considered as bearing a bulky substituent on the other N atom.     

Low-energy excitations of graphene on Ru(0 0 0 1)

The structure and the acoustic phonon branches of graphene on Ru(0 0 0 1) have been experimentally investigated with helium atom scattering (HAS) and analyzed by means of density functional theory (DFT) including Grimme dispersion forces. In-plane interactions are unaffected by the interaction with the substrate. The energy of 16 meV for the vertical rigid vibration of graphene against the Ru(0 0 0 1) surface layer indicates an interlayer effective force constant about five times larger than in graphite. The Rayleigh mode observed for graphene/Ru(0 0 0 1) is almost identical to the one measured on clean Ru(0 0 0 1). This is accounted for by the strong bonding to the substrate, which also explains the previously reported high reflectivity to He atoms of this system. Finally, we report the observation of an additional acoustic branch, closely corresponding to the one already observed by HAS in graphite, which cannot be ascribed to any phonon mode and suggests a possible plasmonic origin.     

Stacking dependent electronic structure and transport in bilayer graphene nanoribbons

The stacking-dependent electronic structure and transport properties of bilayer graphene nanoribbons suspended between gold electrodes are investigated using density functional theory coupled with non-equilibrium Green’s functional method. We find substantially enhanced electron transmission as well as tunneling currents in the AA stacking of bilayer nanoribbons compared to either single-layer or AB stacked bilayer nanoribbons. Interlayer separation between the nanoribbons appears to have a profound impact on the conducting features of the bilayer nanoribbons, which is found to be closely related to the topology and overlap between the edge-localized π orbitals.     

Ultrafast modulation of optical transitions in monolayer and multilayer graphene

We study ultrafast modulations of absorption spectra for both monolayer and multilayer graphene, by performing time-resolved transmission measurements with tuning probe photon energy. While reduced absorptions by photo-excited carriers are observed in monolayer graphene irrespective of the probe energy, multilayer graphene shows increased absorption at around 0.6 eV, which is explained by the optical transitions between subband states. Intraband carrier relaxation and electron–hole recombination times are found to be as fast as 0.5 and 10 ps, respectively. Modifications of ultrafast carrier dynamics are also studied with changing temperature and excitation density.